You have to write some relatively complex law to prohibit it.

For example, Google bought Motorola Mobility -- do they get those patents? What if they then spin off everything, but the patents? Can they do that? Whom do the patents stay with? Can I sell you my company, but not the patents? As an employee, when I grant my employer my patents, have I sold them? Etc...

Because sometimes you have a really good idea, and the ability to make it on a small scale, but you need help getting it perfect and making it big.

In such a case, it's often advantageous to join up with someone else, get investments, start a company, and sell your patents to that company.

That's just one way the ability to sell patents is a good thing.